The present invention relates to the calibration of nuclear density gauges, and more particularly, relates to an improved method of initially calibrating such gauges.
Nuclear radiation gauges for determining density characteristics of soil and asphaltic materials are well known. One example of such a gauge is described in U.S. Pat. No. 2,781,453. Such gauges employ the phenomenon of Compton scattering of gamma rays and are well known to those skilled in the art as "scatter" gauges.
Nuclear density gauges currently in use, for example the Troxler Model 3400 and 4400 series gauges manufactured by the assignee of the instant invention, employ a nuclear source that emits gamma radiation into the test specimen and a detector system for measuring the scattered radiation. The gauge is designed to operate both in a "backscatter" mode and in a "direct transmission" mode, and for this purpose, the source is vertically movable from a "backscatter" position where it resides within the gauge housing to a series of "direct transmission" positions where it is inserted at selected depths into bores in the test specimen.
The counts received by the detector system are found to be related to the density of the scattering medium by an expression of the following form EQU CR=A exp(-BD)+C,
where:
CR=count ratio (the accumulated count normalized to a reference standard count
D=density of test specimen, and
A, B and C are constants.
The gauges are factory calibrated to arrive at values for constants A, B, C for each gauge. The factory calibration procedure which has been used in the past is a time-consuming iterative process which may require several hours to complete. In order to determine values for the three calibration parameters A, B, and C of the above equation, count measurements must be taken on at least three materials of different densities at each source position. In some instances, as many as five calibration standards have been employed in order to take into account the mass attenuation coefficients of different soils. Thus, a large number of individual counts must be taken. For example, for a gauge having a 12 inch source rod with seven different source depth positions, a minimum of 21 separate counts must be taken when three calibration blocks are used. Each count is taken for a period of time, for example, four minutes, with longer periods of time producing greater precision. Once all of the counts are taken, then values for the calibration parameters A, B, and C at each source depth position are calculated. It will thus be seen that this calibration process is a time-consuming and labor intensive procedure.